Rewards: Pounds: 410 (+75), EXP: 250 (+100), Extra: Belt - Dark Leather Belt
Challenges: 1. Kill everyone in the building with the gas.
This memory begins with you standing on top of a building, looking at a factory with a lot of Blighters in front of it. You can take out several Blighters at once by waiting for them to walk underneath the hanging barrels above the front door and throwing a knife at it. There are also hanging barrels from the building you're standing on, and a Blighter will walk underneath them every now and then. There are two stacks of hay down below and you can use either to whistle-lure enemies towards you, taking them out one by one. When there are only one or two remaining, simply take them out regularly with assassinations or combat.
Open the door and follow Darwin around. When he turns the valve, head upstairs and scan your surroundings to spot various Blighters (also on upper levels). There's a female enemy to the left and a brute and thug on the room to the right. First deal with the female enemy on the left; move over the railing and climb to the other side of the area whenever she's in the room, so you can approach her from the back.
Keep in mind: if you want to complete the challenge you CANNOT kill anyone. What IS allowed, however, is strangling people with Circle / B while approaching them stealthily from the back. With her taken care of, move to the nearbly valve on this side of the area and open it. Move to the end of the path and whenever the thug is in the room with the brute, move over the pipe. Wait for the thug to appear and strangle him unconscious. This only leaves the brute in the room. Wait for him to move and approach him from the back (if he doesn't move at all, try whistling, although you need to be fast since the brute goes into suspicion mode). In any case, turn the valve here and move over the pipe to the other side again.
Two more valves need to be opened/sabotaged, and they are on the next level, also located on the central device, directly above the previous two valves. Head back to the room where you strangled the female Blighter and head up the stairs. Scan the area again if you hadn't already. and head up the long stairs to reach the next level. There's a sole thug patrolling at the end of the walkway, but he'll always stop right before the valve (so if you move to the valve, he won't spot you). Turn the valve and wait for the thug to turn his back on you, the strangle him. On the other side of the area, a thug and a brute are moving as a pair. They'll move to the valve on the other side, then into the nearby room and from there to the adjacent room. Wait for them to go to the valve. When they move away into the other room, climb the metal structure in the middle of the area and make your way to the valve. Quickly drop down and turn it, then move to the stairs in the corner (opposite of the enemy pair).
On the next level you'll face a patrolling Blighter pair moving in a circle around the central walkway, a thug moving back and forth on the opposite walkway, and a brute on the left walkway. Your goal is to sneak - unseen - to the switch at the walkway furthest away from you (as marked by the indicator). Wait for the pair to move away and when the coast is clear, either knock out the sole thug Blighter in the building you pass (if he has his back turned towards you) or climb over it. When you make a run for the switch, be sure not to be seen by the brute on the other side. In any case, once you hit the switch, quickly run over the nearby ramp to the right, climbing the chains and pipes to the small window in the distance (which is marked). This is easier than it looks. Congratulations, you've completed the mission and challenge.